The invention relates to a thermographic test method for the locally resolving detection and identification of defects near the surface in a test object and to a testing device suitable for carrying out the test method.
Semifinished products of electrically conductive material, for example billets, bars, rods, tubes or wires of metallic materials, may serve as starting materials for high-grade end products and often have to meet extremely high quality requirements. The testing for material defects, in particular for defects near the surface such as cracks, voids or other inhomogeneities in the material, forms an important part of the quality control of these products. During this testing, it is generally endeavored to test the surface of the material as completely as possible with high local resolution, which wherever possible is carried out as early as possible in the production chain in order to decide on the basis of the result of the test, depending on the type of defects found, whether the defects are uncritical for the further processing, or at least can be repaired by reworking such as grinding, or whether the material has to be discarded.
Apart from the magnetic methods that are often used for such tests, such as the eddy current technique or the stray flux technique, nowadays thermographic test methods are also used for the locally resolving detection and identification of defects near the surface in test objects.
In a known thermographic test method, an electrically conducting test object, for example a steel billet, after rolling runs through an induction coil that is under high-frequency alternating current and induces a current flow near the surface of the test object. On account of the skin effect, which is dependent on the excitation frequency, the current density in the vicinity of the surface of the test piece is greater here than inside the test object. Microstructural disturbances, such as for example cracks, that lie in the cross section of the induced electrical current flow act as electrical resistances and deflect the current flow, which tries to find the path of least (electrical) resistance in the material of the test piece. This results in higher current densities, and consequently also greater power losses, at the “constrictions” of the current flow in the region of the defects. The power loss occurring in the region of the microstructural disturbances is manifested by heat being generated in such a way that the, locally limited, affected region in the direct vicinity of a microstructural disturbance assumes a higher temperature in comparison with the undisturbed surrounding area. With the aid of a heat-sensitive camera or other suitable recording device that is sensitive to thermal radiation, the presence of defects near the surface can then be detected in a locally resolving manner on the basis of the local temperature values within a region of the surface that is recorded by the recording device. A visual representation of the surface regions recorded is generally also provided, and thermographically determined anomalies can be automatically assessed by a downstream evaluating system.
DE 10 2007 055 210 A1 describes a thermographic test method and a testing device set up for carrying out the test method. The testing device has an induction coil for heating up a surface area of a metallic test object running through the induction coil, for example a steel billet, and one or more infrared cameras, in order to measure the temperature profile of the steel billet running through. The results of the measurement are used for activating a color marking system, in order to mark defects that are found. For the evaluation of the thermographic images (thermal images) recorded by the infrared cameras, the description provides evaluation software, which analyzes a thermal image or the thermal images and identifies temperature differences above a predetermined threshold value and reports them as a defect. The size of the temperature difference above the predetermined threshold value is regarded as an indication of the depth of the defect. The evaluation software can evaluate defects both with regard to their length and with regard to the size of the temperature difference above the threshold value. The evaluation software can remove defects of a length below a minimum defect length from a defect list, so that such defects are no longer found to be defects. If, however, a defect lies below a minimum defect length but the size of the temperature difference lies above the threshold value, which lies above a maximum size of the temperature difference, such a defect is nevertheless reported as a defect. In this way, a defect is identified in dependence on the defect length and the temperature difference with respect to the surrounding area.
A rise in the temperature profile of more than 2 K with respect to the surrounding area is generally regarded as a defect, but the threshold temperature may also be chosen to be lower. A temperature difference with respect to the surrounding area of 5 K or more is clearly identified as a defect.
In practice, the temperature profile to be evaluated is generally superposed by an interference signal of an appreciable amplitude. Possible sources of interference include local fluctuations in the degree of emission of the surface of the test object, reflections from the surrounding area and circumstances that are generally unavoidable in actual test operation, such as foreign bodies on the surface of the test piece. False indications may also be caused by the geometry of the test piece, since for example edges on square profiles often have an increased temperature in comparison with the surrounding area. Typically, the temperature differences occurring at a crack-like defect in comparison with the surrounding surface are of the order of magnitude of 1 K to 10 K. It has been observed that interference amplitudes may well also be of this order of magnitude. Therefore, in spite of all possible measures to reduce the interference amplitude, it cannot be ruled out that interferences are falsely classified as microstructural flaws or defects.